The present invention relates to a method and apparatus for combining a magnetohydrodynamic generator and vapor generator to utilize the product gas and the heat output of an oxygen blown coal gasifier to generate power. More specifically, the medium BTU product gas produced in an oxygen-block coal gasifier is used to fuel an MHD burner wherein the high temperature working fluid necessary to generate electricity via magnetohydrodynamics is produced.
In a magnetohydrodynamic (MHD) generator, fuel is combusted to form a high temperature working fluid. The working fluid must be formed at a sufficiently high temperature, preferably in the neighborhood of 2750 C., to promote ionization therein. The working fluid thus formed is passed through a static magnetic field at a high velocity thereby inducing direct current. Ergo, the MHD process provides for the direct conversion of energy into electricity without the need of a conventional turbine generator. When combined with a vapor generator disposed downstream of the MHD generator, wherein the waste heat contained in the gas products in the MHD burner is utilized to generate steam as an auxiliary source of power, overall efficiencies in the area of 50 percent can be obtained.
When integrating an MHD generator with a vapor generator, a number of problems have been encountered to which the present invention is addressed. First, if air is to be used as the oxidizer for the combustion of coal within the MHD burner, an independently-fired air heater must be provided to heat the combustion air supplied to the MHD burner to a high temperature, typically in the neighborhood of 1650 C. Significant technical problems have been encountered in designing high temperature air heaters which lead one to conclude that this equipment will be a high maintenance item and create significant operational problems.
Further, since the independently-fired air heater must utilize a clean fuel in order to prevent fouling and corrosion of heat transfer surface immersed therein, oil or gas must be used. Coal, because of its high ash content, cannot serve directly as a fuel for an independently-fired air heater. Because oil and gas are currently expensive and relatively scarce, this is an undesirable situation.
Second, when air is used as an oxidizer in an MHD burner, very high levels of oxides of nitrogen (NO.sub.x) are formed in the MHD burner from the thermal fixation of nitrogen in the combustion air at the combustion temperatures in the neighborhood of 2750 C. Because the levels of NO.sub.x produced in the MHD burner would be far in excess of the levels permitted to be vented to the atmosphere under present emission regulations, provisions must be made for cleaning the exhaust gases from the MHD burner of NO.sub.x emissions before releasing the exhaust gases to the atmosphere.
The most common method proposed to date for cleaning the MHD exhaust gases of NO.sub.x is to provide a dwell chamber downstream of the MHD channel wherein the exhaust gases of the MHD burner would reside at a high temperature for a long enough time for the NO.sub.x therein to decompose back to molecular nitrogen and oxygen. In order to do this, a typical dwell chamber would be a fairly massive structure lined with refractory material to maintain the exhaust gas temperature above 1600 C. for a period of at least two seconds. To provide such a chamber requires considerable economic expense.
Additionally, economics require that the seed material introduced into the MHD burner to enhance the electrical conductivity of the working fluid discharged into the MHD channel must be reclaimed and recycled. Unfortunately, the high sulfur content of most coals proposed as fuel for the MHD burner results in sulfur contamination of the seed. Thus, expensive reprocessing of the reclaimed seed is required in order to remove the sulfur contamination.